Application of Ni electroplating techniques towards stress-free microelectromechanical system-based sensors and actuators

Abstract

The bimaterial cantilevers are frequently used in microsensors and microactuators. A bimaterial element consists of two layers of materials with different physical properties and/or deposited under different conditions. For this reason, mechanical stresses of various origins appear along the layers and between them, causing initial deformation of the cantilever. Further, a necessary condition for the optimum response of a bimaterial cantilever to an external stimulus is that Young's moduli of both layers are approximately equal. In thermal cantilever-based actuators commonly met in practice, the first layer is usually made of SiO2, PolySi, or SiNx, whereas the second is made of Al or Au. The Young's moduli of these materials differ significantly and thus the maximum sensitivity is difficult to achieve. To ensure the planarity of the free cantilever, the common goal during fabrication is to obtain low internal stresses within both layers, which sometimes represent a severe technological limitation in high-temperature processes. This work presents a new method of stress compensation between the bimaterial cantilever layers. SiO2, PolySi, or SiNx are still used for the first layer, whereas nickel is used for the second layer. Nickel has many advantages and favourable mechanical properties compared with previously used aluminium and gold.